Spray drying is a method of producing dry powder/particles from a slurry or solution liquid by rapidly drying the liquid with a hot gas stream. Spray drying is the preferred method of drying many thermally sensitive materials such as foods and pharmaceuticals. A consistent particle size distribution is a reason for spray drying some industrial products, such as catalysts and other chemicals. Typically, air is the heated drying medium; however, nitrogen may be used if the liquid being atomized is a flammable solvent (e.g., ethanol) or if the product is oxygen-sensitive.
Generally speaking, spray dryers use an atomizer or spray nozzle to disperse a liquid into a controlled-drop-size spray. Common types of nozzle used in spray drying include rotary disc and single-fluid pressure swirl nozzles. Alternatively, for some applications, two-fluid or ultrasonic nozzles may be used. Depending on the process and/or product needs, drop sizes from 10 to 500 micrometers may be achieved with the appropriate choices. However, common applications are often in the 100 to 200 micrometer diameter range.
A hot, drying gas stream (e.g., air, nitrogen, etc.) may be passed as a co-current or counter-current flow to the atomizer direction. The co-current flow method enables the particles to have a lower residence time within the system, and the particle separator (typically a cyclone device) operates more efficiently. The counter-current flow method enables the particles to have a greater residence time in the chamber and usually is paired with a fluidized bed system.
A nano spray dryer offers new possibilities in the field of spray drying. It allows production of particles in the range of 300 nm to 5 μm with a narrow size distribution. High yields are produced—up to 90%—and the minimum sample amount is 1 ml. In the past, the limitations of spray drying were the particle size (minimum 2 micrometers), the yield (maximum around 70%), and the sample volume (minimum 50 ml for devices in lab scale). Recently, minimum particle sizes have been reduced to 300 nm, yields up to 90% are possible, and the sample amount can be as small as 1 ml. These expanded limits are possible due to new technological developments to the spray head, the heating system, and the electrostatic particle collector. To emphasize the small particle sizes possible with this new technology, it has been described as “nano” spray drying. However, the smallest particles produced are typically in the sub-micrometer range common to fine particles rather than the nanometer scale of ultrafine particles. For further information on nano spray drying, see, for example, the Mar. 31, 2011 article entitled “Nano Spray Dryer—Experience Submicron Spray Drying,” available at http://www.labmate-online.com/news/laboratory-products/3/buchi_labortechik_ag/nano_spray_dryer_-_experience_submicron_spray_drying/14005/.
Numerous attempts have been made over the years to improve rotary atomizer performance. For example, U.S. Pat. No. 7,611,069 to Clifford, et al., entitled “Apparatus and Method for a Rotary Atomizer with Improved Pattern Control,” discloses an apparatus and method for forming and controlling a pattern for spraying surfaces with a fluid using a rotary atomizer spray head having an air shaping ring with shaping air nozzles inclined in a direction of rotation of a bell cup to direct the air onto the cup surface near the cup edge. U.S. Pat. No. 7,344,092 to Kim, entitled “Rotary Atomizer, And Air Bearing Protection System For Rotary Atomizer,” discloses a rotary atomizer and an air-bearing protection system for the rotary atomizer to reduce the manufacturing cost. Kim recognizes that high-speed rotation generates a lot of heat and load upon the atomizer during continuous operation. In order to remove this heat, lubricating equipment is commonly used, which leads to complexity in the system structure and consequently to difficulties in maintenance and an increase in the manufacturing cost.
U.S. Pat. No. 6,551,402 to Renyer, et al., entitled “Rotary Atomizer,” discloses a system utilizing a rotary atomizer for applying a liquid-based substance to particles. Renyer recognizes that rotary atomizers typically require a high-speed rotational force within the vicinity of moving particles (as with a continuous flow process) and that machinery that utilizes rotary atomizers can be somewhat complicated, requiring several moving parts which can be subject to frequent breakdowns.
Despite the various advancements in and array of existing atomizers and atomizing systems, current technology still requires regular maintenance and repair, leading to unnecessary repair cost and downtime. Thus, a need exists for an improved rotary atomizer and atomizing system that requires minimal maintenance while yielding increased revolutions per minute (“RPM”) and providing the ability to direct and adjust gas stream velocity.